In order to limit the emission of pollutants from internal combustion engines, especially in motor vehicles, catalytic converters and/or particulate filters or the like have long been used for cleaning the exhaust gas. To ensure that catalytically assisted conversion of the pollutants takes place, the exhaust gas and/or the catalytic converter or the particulate filter must be at a predetermined minimum temperature. Particularly after a cold start or a restart of the internal combustion engine, such a minimum temperature has often not yet been achieved. An attempt is therefore made to increase the temperature of the exhaust gas and/or of the catalytic converter or particulate filter by means of electrically operated heating elements.
Thus, EP-B1-0783621, for example, discloses an electrically heatable catalyst which is constructed with two honeycomb bodies. Here, the first honeycomb body is connected to an electric voltage source and may carry a flow of current. Owing to the ohmic resistance heating, there is then significant heating of the metal foils of the first honeycomb body, wherein the catalytic coating of the first honeycomb body, the coating being in contact with the foils, and/or the exhaust gas flowing through the first honeycomb body is/are heated. For reasons connected with stability, provision is furthermore made for this first honeycomb body to be supported against a downstream second honeycomb body by means of pins and holding elements. Such an embodiment of an electrically heatable catalyst has already proven very useful but still requires a relatively high outlay in production.
Moreover, WO-A1-2013064373 has disclosed an exhaust gas aftertreatment device in which a honeycomb body is provided, on the outlet side of which a receptacle for a heating element is provided. This heating element is formed, in particular, by an electrical conductor, which is surrounded by an insulator. This heating element is formed in the receptacle and is connected to at least one metal layer of the honeycomb body, thus ensuring that the heating element is held captive in the honeycomb body. Such an embodiment of the exhaust gas treatment device makes it simple to produce and to heat electrically. In this case, the heating element is incorporated during the process of producing the exhaust gas aftertreatment device, but it is also possible to incorporate the heating element into the honeycomb body after the production of the body. However, this concept has the effect that selective machining of the metal layers of the honeycomb body is required and that the exhaust gas is heated only a short time before exiting from the honeycomb body through contact with the heating element.
In motor vehicles, a predetermined energy or voltage is often only available at certain times. In many cases, therefore, it was previously only possible to operate such electric heating elements at 12 volts or a maximum of 24 volts. Now, however, there is the possibility of onboard electrical systems being able to deliver up to 48 volts. To achieve adequate ohmic resistance heating in this case, the electric heating element must form a relatively high electrical resistance. In this context, there is a preference for embodiments of the heating elements which provide an electrical resistance in a range of from 0.5 to 5 ohms. The application or use of 48 V energy systems or onboard electrical systems poses new challenges to the design of the electric heating elements and the incorporation thereof into exhaust gas aftertreatment systems. On the one hand, this concerns the operation or arrangement of such electric heating elements in the exhaust gas aftertreatment system and, on the other hand, the manufacture of corresponding honeycomb body arrangements, which should be embodied in a simple manner in view of costs and assembly.
Corrosion has proven problematic in connection with the use of high-voltage heating elements (e.g. 24 volts or especially 48 volts). Thus, for example, it has been found that the corrosive loss of the materials for the heating conductor (e.g. the metal foils of an electrically heatable honeycomb body) increases with rising voltage. Thus, it has been found that this corrosive loss of a range of materials rises by a significant factor F when operating with a 48-volt network as compared with a conventional 12-volt network, wherein F is at least 3, at least 5 or even at least 8 (determined in a corrosive environment, namely with NaCl wetting). Consequently, there is a considerable requirement for adaptation with a view to long-term operation of such heating elements in this environment.